The present invention relates to a process for regenerating and processing (further condensing; working up), an inactive, linear polyalkylene terephthalate having a viscosity number of at least 60 cc/g, in two stages, at an elevated temperature, in the solid phase, and in the presence of alkanediols.
Polyalkylene terephthalates, especially polyethylene terephthalate (PETP) and polybutylene terephthalate (PBTP) are excellent materials for the production of films, sheets, molded parts, etc. Especially for the manufacture of molded parts by injection molding and extrusion processes, polyalkylene terephthalate must exhibit a viscosity number of &gt;100 cc/g.
Polyalkylene terephthalates are customarily produced by interesterification or esterification and subsequent polycondensation of terephthalic acid or its polyester-forming derivatives and an alkanediol, in the presence of catalysts (Sorensen and Campbell, Preparative Methods of Polymer Chemistry, Interscience Publishers Inc., N.Y. [1961]: 111-127; "Kunststoff-Handbuch" [Plastics Manual] VIII, C. Hanser Publishers, Munich [1973], whose disclosures are incorporated by reference herein.)
Using these processes, carried out in the melt, viscosities of a level required, for example, for injection molding and extrusion purposes, cannot be attained. Due to the temperature load, degradation reactions occur as early as the polycondensation stage, so that the desired high viscosity cannot be achieved.
To attain the desired, high viscosities, a solid-phase recondensation is, therefore, added to the process (British Pat. No. 1,066,162; U.S. Pat. No. 3,405,098). This is done by bringing the polyester, in granulated or powdered form, to temperatures lying approximately 10.degree.-60.degree. C. below the melting point. In this temperature range, the polyalkylene terephthalate is treated in an inert gas stream or under vacuum until the desired, high viscosity has been attained.
During this recondensation in the solid phase, the problem frequently occurs that a batch can be condensed only very gradually and only up to a viscosity lying below the desired, high level. Such a polyalkylene terephthalate is termed passive or inactive. (See DAS [German Published Application] No. 1,570,689, whose disclosure is incorporated by reference herein).
According to the state of the art, these inactive polyalkylene terephthalates are degraded by alcoholysis or hydrolysis back to low-molecular weight components which are then polycondensed in a new batch (DOS [German Unexamined Laid-Open Application] No. 1,420,366). According to the latter reference, for example, polyethylene terephthalate is degraded with dimethyl terephthalate and ethylene glycol in the presence of catalysts to form a low-viscosity interesterification mixture.
DAS No. 1,570,689 proposes to conduct the recondensation of polyethylene terephthalate after a treatment with ethylene glycol. In this process, the product must be soaked over several days in ethylene glycol. During the removal of excess ethylene glycol at a temperature above 230.degree. C., the material tends to cake. If the ethylene glycol treatment is conducted at a high temperature (DAS No. 1,570,689, Example 3) in order to shorten the treatment time to a technically practicable length, the resultant volatility of ethylene glycol is too high to produce a reactivation of batches on a technical scale.